In recent years, mobile communications have made startling progress along with a remarkable development in the information industry. This will lead to the introduction of a ubiquitous sensor network that allows a user to communicate whenever, wherever, and whatever to whomever he or she likes. In order to embody such a ubiquitous sensor network, a large quantity of communication devices is installed on the surface of the earth or moves around to perform a communication function. The communication device should always be driven without an external wire power source or disposable battery, regardless of where it is.
In particular, a mobile communication device can make use of a solar battery as an energy source. A silicon-based solar battery is the most popular conventional solar battery. The silicon-based solar battery has a photoelectric conversion efficiency of less than 10% and a silicon material has a low bandgap of about 1.1 eV. For these reasons, several batteries should be connected in parallel to obtain an optimum voltage. Accordingly, it is difficult to apply the silicon-based solar battery to the communication device. To solve this problem, a solar battery using a compound semiconductor has been proposed.